Run update packages after fetching deb w/o source

- If we don't find a package's source, we fetch it from the last build's
package tarball. In the event that it was the last package fetched,
nothing would have updated the apt repo, so installing would fail at
rootfs time. Run update_packages.sh after fetching from the prebuilt to
ensure that these packages are available.

Change-Id: I5e190bec70a100c2dc8b4a41afb429a3aa84f986
diff --git a/packages.mk b/packages.mk
index 193bf3d..d901402 100644
--- a/packages.mk
+++ b/packages.mk
@@ -78,6 +78,7 @@
 	tar -C $(PRODUCT_OUT) --wildcards -xf \
 		$(PACKAGES_FETCH_ROOT_DIRECTORY)/$(PACKAGES_REVISION)/packages.tgz \
 		packages/$1*.deb
+	$(ROOTDIR)/build/update_packages.sh
 endif
 	touch $(PRODUCT_OUT)/.$1-pbuilder
 .PHONY:: $1